Technical Field
The present invention relates to a toner, an image forming method, and a process cartridge.
Background Art
Electrography employed in image forming apparatuses such as laser printers and dry type electrostatic photocopiers includes the following steps:
1. Uniformly charging the surface of an image bearing member such as photoconductive layer;
2. Exposing the surface of the image bearing member to light to form an electric latent image by erasing charges on the exposed portion;
3. Rendering the latent image visible by attaching fine powder having charges such as toner to the latent image;
4. Transferring the obtained visible image to a recording medium such as a transfer sheet followed by application of heat and pressure to permanently fix the image thereon; and
5. Removing fine powder that has not been transferred but remained on the surface of the image bearing member.
To apply heat, a heating device such as a heat roll, an oven, and a flash is used.
Heating temperature is controlled by a thermostat or other sensors.
The image forming apparatus of late have been demanded to enjoy energy efficiency and high performance. Accordingly, toner is demanded to have properties of melting and fusing at low temperatures.
Although low temperature fixing is made possible by just lowering the melting point of toner, the storage stability of the toner becomes a concern.
In addition, demand for better image quality is strong. Specifically, clear and vivid gloss is demanded for high grade images such as photographs.
Furthermore, in the fixing method by heating as described above, to conduct fixing by heating by, for example, a heat roll, the surface temperature of the heat roll is controlled considering the characteristics of toner used. In such a case, the surface temperature of the heat roll changes depending on operation and suspension of the heat roll, the passing state of a recording medium, environment conditions, overshooting of the heat roll, etc. Therefore, high gloss is required to be achieved irrespective of the change of fixing temperature.
As methods of forming gloss images on the same recording medium in electrophotography, a method of controlling gloss by the number average molecular weight of a resin for use in toner is disclosed in JP-H8-220821-A, a method of improving releasability during fixing is disclosed in JP-2003-5432-A, and a method of controlling gloss by adjusting the viscoelasticity of transparent toner is disclosed in JP-2011-100106-A. In addition, JP-2009-217083-A discloses a method of imparting gloss by softening transparent gloss controlling particles during fixing to smooth the surface of an image.
As described above, there are various methods to control gloss on a recording medium. For example, JP-H8-220821-A mentioned above uses a polyester resin having a number average molecular weight of about 3,500 for a transparent toner and a polyester having a number average molecular weight of about 10,000 for a colored toner. The melting point of the transparent toner is lower than that of the colored toner, thereby increasing smoothness, so that the gloss of the portion of the transparent toner is partially improved.
However, the transparent toner is applied as the uppermost layer of an image so that it directly contacts a fixing device. Accordingly, the transparent toner is required to have a better hot offset resistance than the colored toner. Moreover, since the transparent toner is overlapped on the colored toner image, the toner layer becomes thick. Unless the colored toner has a good cold offset resistance, the results of such a combination of a transparent toner having a low melting point and a colored toner having a high melting point tends to be unstable.
To impart a good hot offset resistance to toner, a cross-linkable monomer is in general introduced into a resin to obtain a wide molecular weight distribution, thereby preventing occurrence of hot offset.
However, if such a cross-linkable monomer is introduced, hot offset can be prevented but fluidity is not demonstrated because of elastic components. As a result, the smoothness of the surface of toner is impaired, thereby lowering the gloss of an obtained image.
In addition, JP-2003-5432-A mentioned above discloses that a styrene-acrylic resin is used as a polyester resin to disperse a releasing agent in order that the releasing agent becomes a suitable size to demonstrate releasability, meaning that the adverse impact of the releasing agent contained in the toner is lessened. Furthermore, by using a particular acrylic resin for the polyester resin, decrease of the gloss of an image can be subdued.
However, spot high gloss close to photograph gloss obtained by spot varnish is not realized yet.
In addition, JP-2011-100106-A mentioned above discloses that high gloss can be demonstrated under the condition that the loss tangent (tan δ), which is represented by the ratio of loss elastic modulus (G″) to storage elastic modulus (G′), has the maximum peak in the range of from 80° C. to 160° C. with a maximum peak value of 3 or more.
However, JP-2011-100106-A mentioned above does not mention whether the fixing temperature showing high gloss has a range.
Moreover, in the method disclosed in JP-2009-217083-A mentioned above, since the melting point of a material to soften a binder resin of the gloss control particles, the storage stability of toner is not sufficient.